The present invention relates to improvements in a sprocket wheel for use in a photographic camera.
An involute curve is applied to the tooth form which is used in each tooth of a sprocket wheel for a conventional camera, in order to reduce sliding friction between the film perforations and the teeth. Although this conventional tooth form can generally be considered useful in that the transmission of rotative motion is conducted smoothly, the fact that the amount of sliding friction associated therewith is small sometimes presents a disadvantage in certain kinds of cameras.
More particularly, this adverse phenomenon occurs in cameras wherein the range of meshing angle between the film perforations and the teeth on the sprocket gear is relatively small.
Generally, in the so-called 35 mm frame size (36 mm.times.24 mm) camera, sprocket wheels having a large outside diameter and 8 teeth arranged with equal angles about its outer periphery so that the rotation of the sprocket wheel necessary to advance one frame is exactly 360.degree., or having a small outside diameter (as compared to the former example) and 6 teeth whereby the rotation thereof necessary to advance the film one frame is greater than 360.degree. so that the size of the camera is small, are known. In the former case, even when the sprocket wheel meshes with film disposed in a straight line, the range of meshing angle between the sprocket wheel teeth and the film perforations is broad due to the relatively large outside diameter and the many (8) teeth, and advancing of the film is therefore conducted smoothly in the film winding operation. However, in the latter case, the range of meshing angle between the sprocket wheel teeth and the film perforations is notably narrow. During sprocket wheel rotation in this latter case, therefore, the film tends to rise or be lifted up along the tooth profile as a result of the aforesaid reduced sliding friction between the film and engaging teeth to such an extent that the film perforation sometimes disengages completely from the sprocket gear. Such phenomenon is especially pronounced in cameras in which the leading edge of the film inserted into the camera is fed into the film take-up chamber by the winding or feeding motion of the sprocket wheel alone, and in automatic film loading cameras in which the leading edge of the film is wound round the take-up spool by the feeding motion of the sprocket wheel and the film transporting resistance is great or the placement of the film guide roller promotes the problem. As a consequence, in cameras in which the film winding motion by the sprocket wheel is effected by a motor actuated when the back lid of the camera is closed, it sometimes happens that the film is not actually being wound or advanced--unbeknownst to the user since it cannot always be determined from outside of the camera whether the film has riden up along the tooth profile and disengaged completely from the rotating sprocket wheel.